1. Field of the Invention
The present invention relates to an image pickup apparatus and exposure control method. More particularly, the present invention relates to an image pickup apparatus and exposure control method in which an exposure value can be determined precisely even in a short time in a structure with AE control and AF control.
2. Description Related to the Prior Art
A digital still camera is an image pickup apparatus widely used, and includes an image sensor of a CMOS type or CCD type. An object image is photographed by an image sensor, is converted into digital image data, which is recorded in an internal memory, memory card or other recording medium. A type of digital still camera is widely used in which auto exposure (AE) control and autofocus (AF) control are carried out by referring to an image signal obtained by the image sensor.
In the AE control, object brightness is detected from the image signal obtained by photographing of an object image by an image sensor. An exposure value, namely a combination of a shutter speed and an aperture stop value, is determined to optimize the exposure according to the detected object brightness. In the AF control, a contrast detecting method is widely used. When a contrast of an object, namely an integrated value of a high frequency component of an image signal obtained by exposure of an object image, becomes the highest, the position is obtained as an in-focus position. The objective lens system is moved and set in the in-focus position.
For the AF control, the contrast is evaluated while an in-focus position of a lens system is moved serially, to detect a position where the contrast is maximized locally. However, only insufficient contrast is obtained from an object image with a low brightness level, for example, in an indoor place with little illumination. There is a digital still camera in which AF auxiliary light is applied to an object from a light source, to raise the object brightness.
In the AF control, an exposure value according to the brightness level which is detected shortly before is used in a fixed manner. If the brightness level changes with AF auxiliary light, the exposure value becomes improper, to lower the precision in detecting the in-focus position. To solve such a problem, U.S. Pat. No. 7,145,602 (corresponding to JP-A 2003-008985) discloses an exposure control in which exposure amounts are obtained before and after application of AF auxiliary light. If an increase in the exposure amount after using the AF auxiliary light is equal to or higher than a predetermined amount, the exposure amount before using the AF auxiliary light is used. If the increase in the exposure amount after using the AF auxiliary light is lower than the predetermined amount, the exposure amount after using the AF auxiliary light is used. JP-A 2003-114374 discloses a digital camera in which a difference in the object brightness is obtained between a state before illumination of AF auxiliary light and a state during illumination with a highest light amount. An optimized light amount of AF auxiliary light is determined.
A dynamic range of an image sensor is small owing to its characteristic. A range of the object brightness measurable at one time is small with the image sensor for light reception. To measure the object brightness with precision, photometry is effected for a plurality of times at different exposure values in combination of the shutter speed and aperture stop value. However, it is likely that a great delay occurs until detection of an in-focus position if the photometry of a plurality of times is effected before the AF control. In particular, if the measurement is directly before the AF control for exposure of a still image in response to a shutter release button, there arises a problem in which time lag of the shutter release is too great in a period from the depression of the shutter release button to forming of a still image by exposure.